The present invention relates to an adhesive composition according to the pre-characterising part of the main claim, which contains inductively heatable filler particles, and to its use and a process for its curing. The invention also relates to an adhesive composite that contains a hardened layer of the adhesive composition, a process for the thermal dissociation of the hardened adhesive composition, and the use of this process.
Adhesive bonds, i.e. in particular bonded joints, coatings, laminates or cast structural parts are designed so that they can be produced under mild conditions, are resistant for as long as possible and have the highest possible strengths. High strengths mean that, in the case of a repair or recycling, a dissociation of the adhesive bond can be carried out only under extreme conditions, such as for example the action of strong forces or high temperatures. Bonded joints based on hard adhesives are generally dissociable, but are not suitable for transmitting the high forces necessary for structural bonded joints. The dissociation of high-strength bonded joints is generally accomplished by the use of mechanical energy or chemical agents. The latter have the disadvantage that they cause a high environmental pollution and also that the penetration of the agents into the gluelines of structural bonded joints that are stable over the long term takes far too long.
DE 43 28 108 A describes the dissociation of floor coverings by means of microwave energy. For this purpose a contact adhesive is used that is electrically conducting and is filled with copper powder or aluminium powder. These fillers have the disadvantage that the particles have sizes of a few micrometers and larger. This leads to a non-uniform heating of the contact adhesive.
DE 199 61 940 A1 describes adhesives for dissociable bonded joints that contain thermally activatable substances that release gaseous compounds when they decompose, which then destroy the bonded joints. This process has the disadvantage that in order to separate the composite, the whole structural part or the joined parts and the adhesive have to be heated. This is associated with a high energy expenditure. Furthermore it is not possible to achieve a locally restricted separation of the structural part or of the joined part.
DE 199 51 599 A1 and DE 199 24 138 A1 describe adhesives for dissociable bonded joints and bonded joints produced therewith, that contain externally excitable nanofillers. The dissociation of the bonded joints is achieved by introducing the joints into an alternating electrical, magnetic or electromagnetic field, whereby the nanofillers and the surrounding adhesive are heated. This process has the disadvantage however that it leads to the heating of the whole adhesive, also at places at which no heating is necessary or desirable, since the excitable nanofillers are also contained in places in the adhesive or primer where heating is not necessary in order to achieve the desired dissociation of the bonded joint. Furthermore high temperatures are required to separate high-strength bonded joints since chemical bonds have to be broken in order to break down the composite. The described processes furthermore have the disadvantage that a non-specific thermal decomposition of the adhesive and/or primer occurs when separating high-strength bonded joints. Such processes are therefore unsuitable in particular for thermosets.
The production of resistant, high-strength adhesive joints is normally carried out thermally or photochemically. The conventional processes for producing adhesive bonded joints have the disadvantage however that the whole structural part has to be heated in order to cure the adhesive. As a result the process is energy intensive and time-consuming.
WO 99/03306 and O. Hahn, A. Kaimann in Adhäsion—Kleben und Dichten, 10/2001, pp. 35-38 describe a process for the inductive curing of adhesive joints. In this case adhesives that contain inductively activatable fillers are introduced into an electromagnetic field, whereby the inductively activatable fillers are heated and the hardening of the adhesive surrounding the fillers can take place. These processes have the disadvantage however that the inductively activatable substances are not uniformly distributed over the adhesive and accordingly there is an inhomogeneous heating of the adhesive. As a result, the strength of such adhesive joints is limited. The processes furthermore have the disadvantage that a demixing (separation) may occur in the adhesive during the inductive heating process and the distribution of the thermally activatable substances in the adhesive becomes even more irregular.